Consider these things as mnemonics: They’re and you’re are easy to distinguish from the other words if you realise that the ‘re part is short for are. Same with the ‘s in it’s being short for is.
The i in their can be seen as an assimilation of the y in they.

Oh, and a trick for their and there, is the one with the “i” in it is about people. That makes it easy for me to remember, because when I write about myself I use I all of the time and I am a person. There is about a place. They’re is obvious again for the contraction rule I just mentioned above.